Steamiest NC-17 Flims: From Midnight Cowboy to Blue Valentine

Blue Valentine, an already critically-acclaimed film generating Oscar buzz, won't be released until the end of the year, and if the MPAA gets its way, it likely won't be showing at a theatre near you.

The Motion Picture Association of America rated the rollercoaster romance NC-17, restricting audiences to adults over 17, and guaranteeing that many cinemas across the country will choose not to include the movie on their marquees.

The NC-17 rating, once known more ominously as "X," can – and will, according to the film's producers, the Weinstein Co. – be appealed. Alternatively, the film can be edited, reviewed again and given a less restrictive rating, or the filmmakers can forgo an MPAA rating and sell the film to theatres without a rating.

The MPAA won't confirm why the movie received the rating it did, but the indy romance, directed by Derek Cianfrance and staring Ryan Gosling and Michelle Williams, follows the couple's tumultuous relationship and includes a drunken sex scene in a hotel room.

"We want to express our deepest gratitude to our colleagues in the industry and in the media for their recent outpouring of support for Derek Cianfrance's 'Blue Valentine' after the film surprisingly received an NC-17 rating from the MPAA," Weinstein Co. chief Harvey Weinstein said in statement. "We are taking every possible step to contest the MPAA's decision.

"We respect the work of the MPAA and we hope, after having a chance to sit down with them, they will see that our appeal is reasonable, and the film, which is an honest and personal portrait of a relationship, would be significantly harmed by such a rating."

The MPAA replied with a statement to ABCNews.com:

"It's not our policy to comment on individual films until the rating has been accepted, and then only to state the rating and the reason for the rating.

"Every film is screened by a group of raters who are parents, who apply the standard of what the majority of American parents would rate the film. As part of the classification and ratings process, when a rating is offered every independent filmmaker has the option of accepting the rating, appealing the rating, editing the film and resubmitting it or releasing the film unrated.

Rated NC-17: Box Office Death for 'Blue Valentine'?

"If they choose to appeal the rating, the appeals board is composed of distributors, exhibitors, and industry people who will either uphold or overturn the rating."

While a final NC-17 rating can spell paltry sales at the box office, the whiff of a preliminary NC-17 rating – with its promise of sex and violence -- can increase interest in a movie.

In chronological order, ABC News.com presents some of the most popular films to ever earn the NC-17 rating:

'Midnight Cowboy'

Released in 1969, before NC-17 became an official MPAA rating, "Midnight Cowboy," starring Dustin Hoffman and John Voigt, became the first X-rated film to earn an Academy Award.

The film, which follows the story of a struggling male hustler and his conman friend struggling to get by in New York City, earned Oscars for Best Picture, Best Director and Best Adapted Screenplay.

The film, which includes a scene referencing gay sex, earned an X rating and opened to limited release in 1969. Following its Academy Awards coup, the MPAA revised the film's rating to R, meaning parents could accompany minors to screenings when the film was re-released in 1971.

'The Wild Bunch'

Also released in 1969, when "The Wild Bunch" hit theatres it bore the more inclusive R rating. It is often ranked among the best American movies ever made.

But in a strange twist of ratings fate, when it was rereleased in 1993 – three years after the MPAA changed its ratings protocol – it was branded with an NC-17.

The film, about a gang of outlaws in 1913 Texas, stirred controversy for its gory depictions of violence and death.

'Last Tango In Paris'

1972's romantic drama "Last Tango in Paris" starred Marlon Brando as an aging American widower who has a lengthy anonymous sexual tryst with a young Parisian woman.

The film, which ends in murder and accusations of rape, was given an X rating. ABC News's Harry Reasoner at the time called the film "pornography disguised as art."

A 1981 edited version of the film got an R rating, but when the film was rereleased in 1997 it was given an NC-17.

'Showgirls'

One of the most panned movies ever produced, 1995's "Showgirls," starring Elizabeth Berkely, received an NC-17 rating for "nudity and erotic sexuality throughout, some graphic language and sexual violence." In addition to numerous nude scenes the film also depicted drug use and a rape.

Labeled a sexploitation film, the film follows Berkely's character Nomi as she tries to become a Las Vegas showgirl. The film was so hyped, United Artists sent employees to staff movie theatres across the country to ensure underage viewers did not try to sneak into see the picture.

'The Dreamers'

A small-budget art film, "The Dreamers," about an American student's sexual relationship with a French brother and sister set in 1960's Paris, earned a respectable $2.5 million in U.S. box office receipts despite its NC-17 rating.

Starring Michael Pitt and released in 2003, the film depicted incestuous relationships, threesome sex and male frontal nudity.